Uncertainties in the temperature dependence of the line-coupling parameters of the microwave oxygen band: impact study

A sensitivity study was performed with the commonly used millimeter-wave propagation model (MPM) to assess the impact of prescribed uncertainty in the temperature dependence of the line-coupling factors used to model the oxygen line shape. In some cases, apparent nonphysical behaviors were noticed and their impact on microwave channels was found to be significant and more importantly, nonremovable by simple calibration or bias removal. The Advanced Microwave Sounding Unit configuration in particular was tested because of its prime importance in current data assimilation. This impact, which reached a maximum of a few Kelvin in some channels, is modulated nonlinearly by the amount of water vapor present in the atmosphere, with maximum impact in dry air situations, which makes it dependent on the brightness temperature itself. These errors directly impact the error budget of any physically based geophysical retrieval. The line shape of these O₂ lines has impacts across the microwave spectrum, through the wing effects. This includes channels near 23, 36, and 85 GHz, commonly used in operational radiometry.